Field of the Invention
The invention relates to a folder for rotary printing presses, in particular, a pinless folder, and a method of operating the folder.
A folder for a printing press has been disclosed in the prior art, as exemplified by the published German Patent Document DE 44 46 753 A1. The folder is made up of a first folding cylinder provided with controllable grippers, and a second folding cylinder likewise provided with controllable grippers. The second folding cylinder is provided with additional controllable grippers, besides the controllable grippers already present on the circumference thereof, in order to press the trailing end of signatures against the circumference of the folding cylinder. The additional controllable grippers, respectively, are formed of a spring-steel blade which is held by a bearing connected to a transversely disposed tube. The additional controllable grippers, respectively, cooperate with a holder carrying a bar formed of elastic material.
The published German Patent Document DE 43 42 037 C1 discloses a method and a device for cross-folding signatures. In a folder which is arranged downline of a rotary printing press and has a collecting cylinder, the signatures are cross-folded in half in the initial region of a first and leading half, respectively, of signatures, the respective first and leading half being retained by holding devices. This occurs by having a folding blade penetrate into folding jaws of a jaw cylinder, copies which have been folded in half being retained in the folding jaws. Then, in a region of the point of intersection of an imaginary plane connecting the axes of rotation of the cylinders and a tangent to the circumference of the collecting cylinder, a respectively second and trailing half of the signatures is gripped, and secured, in an end region thereof by a controllable securing device. Thereafter, respectively, the initial region of the first and leading half of the signatures is released by the retaining devices of the collecting cylinder at the latest when the latter is at zero speed, or near zero speed, until the beginning and the end of the signatures retained in the folding jaws, respectively, are located more or less above one another. Finally, the securing device of the jaw cylinder again releases the trailing and second half, respectively, of each copy.
The problem of a so-called whiplash effect arises in cross-folding operations. The high accelerations which occur during transfers of copies from one cylinder to the other result in uncontrolled spreading and fanning out, respectively, of the end of the second, i.e., trailing, half of a signature; a folding-over or dog-earing of corners is also possible. In the solution to this problem set forth in the aforementioned published German Patent Document DE 43 42 037 C1, only part of the problem is solved, because the other end of the signature remains free. In previous attempts to find a solution, the problem has also occurred that an outwardly extending movement of the copy-gripping grippers into the nip between copy-guiding cylinders has always resulted, during a relatively longer period of rotation of the cylinders in the circumferential direction, in an increase in size of the enveloping curve around the relevant cylinder.